1. Field of the Disclosure
This disclosure pertains generally to microfluidic devices and pumping schemes, and more particularly to an apparatus and system for microfluidic pumping where the sample fluid is isolated from the pumping mechanism. Two channel networks, the fluidic channel and the degas channel, are located in close proximity to each other and an air concentration gradient created across the two networks generates an in-situ diffusive flux out of the fluidic channel resulting in a fluidic channel pressure reduction causing movement of the fluid.
2. Discussion
Microfluidic technologies promise to significantly improve chemical and biomedical analysis by enabling complex laboratory analysis within highly efficient and disposable devices while minimizing fluid volume and power requirements. However, very few of the advancements in microfluidic technology have been successfully commercialized into functional and practical products for point-of-care testing and on-site environmental monitoring. An essential challenge in accomplishing functional microfluidic devices is the need for reliable fluidic actuation for bubble-free, reliable, stand-alone, autonomous loading of fluids.
For rapid testing applications, the mechanism of fluid actuation of the sample has a significant influence on downstream component performance. Current attempts at integrating biochemical assays into microfluidic systems often require fluid actuation methods with several pumps and reagent reservoirs. Therefore, power-free and portable actuation of biofluids in microfluidic chips is essential to the creation of point of care microfluidic diagnostic devices.
Although many fluidic actuation schemes have been developed for microfluidic devices, most of these methods are either excessively complex, dependent on external power sources eliminating mobility, or are overly simplified and not amenable to the controlled and complicated flow patterns required for the integration of advanced biomolecular assay techniques. The majority of the various pumping methods for fluid transport can be classified as either active or passive fluidic actuation techniques. However, both types of fluidic actuation techniques have systemic limitations.
Pressure-driven pumping can be achieved by either positive or negative pressure, which pushes or pulls fluids through microfluidic systems. The most common techniques used for active fluidic actuation in microfluidic systems are pressure-driven or electrokinetic pumping. These techniques offer a range of flow rates that can be controlled. However, pressure-driven actuation often requires burdensome or expensive external equipment and the scheme is vulnerable to bubble formation that can create large dead volumes as well as oscillatory flow. Additionally, these active pumping mechanisms are generally not suitable for filling up dead-end structures that are attractive for complex quantitative, digitized and multiplexed assays.
The same problems are encountered with the electrokinetic-driven flow, which requires a high voltage (50 V to 1000 V) to be exerted on the integrated electrodes to transport the liquid. This high voltage requirement makes it difficult to miniaturize without off-chip power supplies, and the flow control is not easy due to the variability in material surface properties and the vulnerability of fluid properties. Additionally, air bubbles and heat can develop in electrokinetic flow systems, interfering with liquid manipulation and system operation.
Although these active fluid actuation methods have advantages that make them desirable in some applications, their characteristics restrict their utility in applications where portability is essential, thus use beyond the research laboratory has been limited.
Passive fluid actuation methods have been popular due to their simplicity and reduced dependence on external equipment. The common passive actuation techniques include capillary, gravitational and evaporation-based pumping. Capillary-driven flow is a widely adopted and successful autonomous fluidic actuation method utilized in Lateral Flow Assays (LFAs) and glucose measurement tests. Capillary-driven flow is advantageous when the specific liquid-gas-solid condition can provide sufficient capillary pressure necessary to load the liquid autonomously. However, this method is only used in non-complex, low accuracy applications because it lacks controllability and repeatability in humid settings.
Gravity-driven, evaporation-driven, and droplet-based flow techniques take advantage of physical phenomena, and are highly dependent on environmental conditions that greatly influence reproducibility and reliability. Gravity-driven flow is based on the physical existence of gravity but the devices need pre-priming and sometimes the surface tension dominates in the miniaturized devices leading to inconsistent flows. For the pumping based on surface tension, such as evaporation-induced and droplet-based driven flow, the vulnerability to environmental conditions (temperature, humidity etc.) restricts them to applications and uses at locations that have controlled atmospheric conditions that do not vary during the operation process.
Although passive devices are portable and disposable, these passive actuation techniques often lack tunability, reproducibility and require meticulous device pre-treatment or storage, which limit their employment in devices requiring accuracy and reliability.
Additionally, most of the current actuation techniques involve the direct exposure of the liquid sample to the actuation apparatus. Apart from causing problems with air bubble formation, fluid instabilities, dead volumes and inter-device repeatability, direct exposure of biofluids to the actuation apparatus raises concerns about biosafety especially when dealing with biohazardous fluids.
Accordingly, there is a need for an apparatus and method that has tunable, reproducible, and bubble-free microfluidic pumping without any auxiliary equipment or device pre-treatment and that can fill dead-end channels and chambers for a broad range of applications.